The present invention relates to a mobile or portable telephone set and, In particular, to level control of a sound signal in such a portable telephone set.
In the known portable telephone set or a mobile telephone set such as terminals in the Cellular Communication system, the Personal Handy-phone System and other wireless communication system, a sound signal inputted by a microphone is amplified by an audio-frequency amplifier. The amplified sound signal is digitized and coded by a codec into a digital sound signal which is carried on a carrier frequency signal and is transmitted as a wireless signal.
The sound signal has different sound levels dependent on sound pressure of the utterance to the microphone. In the known portable telephone set, the sound level of the sound signal is not adjusted before transmitting therefrom. At receiving party, the sound level is only adjusted by a volume of sound.
However, when the sound pressure of the utterance is excessively high, distortion is caused at the audio-amplifier and/or an overflow is caused at an analog/digital converter in the codec. As a result, sound is distorted at the receiver side.
On the contrary, when the sound pressure is weak, the volume adjustment must be performed at the receiver side.